Various types of humidity sensors have been developed over the years. A cost-effective miniaturized solution is to employ a solid state sensing film that interacts with water vapor to convert the water vapor concentration into an electrical signal. Techniques for converting the water vapor concentration into electrical signals include capacitive sensing, thermoelectric sensing and magnetoelastic sensing. A typical electrical humidity sensor may sense water vapor concentration, by having a capacitance or impedance between two electrodes. The capacitance of impedance may vary with the relative permittivity of a polymer film. The relative permittivity of the polymer film may be sensitive to an amount of water vapor absorbed by the polymer film. The typical accuracy of this type of sensors may range from ±1.7% to ±5%, which can meet the requirements in some consumer electronics, for example for use in environmental sensing and medical devices for breath testing. Another method for measuring relative humidity may be to use a “chilled-mirror” dew-point hygrometer. A “chilled-mirror” dew-point hygrometer may include a miniature polished mirror that may be cooled by a Peltier thermoelectric heat pump until the mirror reaches the dew-point of the water vapor under test. When the dew point temperature has been reached, condensation will begin to form on the mirror surface. The “chilled-mirror” dew-point hygrometer may include an electro-optical loop to detect a reduction of the light intensity reflected from the mirror surface and thereby, detect that condensation. The typical accuracy of such a dew-point hygrometer may be ±0.1° C. according to relative humidity accuracy of ±0.014% at a relative humidity of 1%. The high accuracy of a “chilled-mirror” dew-point hygrometer may allow it to be applied to a range of high-end applications including metrology laboratories, aerospace, natural gas, petrochemical and meteorology. However, there are many barriers to using the “chilled-mirror” dew-point hygrometer, including its high cost, large size and slow response rate. Other potential disruptive solutions for high accuracy detection of relative humidity include carbon nanotubes and graphene which exhibit very high humidity sensitivity. However, such solutions still require further development in order to be understood and applied to practical applications.
As such, there is a need for a humidity sensor that can achieve high accuracy with a relatively fast response rate, while being relatively small size and being available at a low cost.